1. Field of the Invention
The invention relates to a maraging steel. The invention steel preferably contains no cobalt or titanium, and has a high elastic limit and good resistance to fatigue.
2. Background of the Invention
Maraging steels are self-tempering steels which can acquire a soft martensitic structure by cooling in air, which structure can be appreciably hardened by a thermal aging treatment which gives rise to formation of intermetallic precipitates. In essence, these steels contain:
10-30 wt. % nickel, which enables one to obtain a martensitic structure by cooling in air; PA1 a low carbon content which enables one to obtain a soft martensite; and PA1 additional elements which enable hardening by formation of intermetallic precipitates, said elements being namely titanium, aluminum, and molybdenum, as well as cobalt, where the presence of the cobalt enhances the effects of the other added elements. PA1 Ni 14-23 wt. %, Mo 4-13 wt. %, PA1 Al 1-3.5 wt. %, C.ltoreq.0.01 wt. %, (all based on total weight), PA1 iron and impurities resulting from the processing; PA1 where the composition preferably also satisfies the following conditions: PA1 Ni 14-23 wt. %, preferably &gt;16 wt. %, and PA1 Mo 4-13 wt. %, preferably 5-8 wt. %.
One may also add niobium, to fix the carbon and thereby soften the un-aged martensitic structure.
These steels were devised in the face of the problem of simultaneously obtaining a very high limit of elasticity and good ductility. Initially, good ductility was obtained by simultaneous addition of cobalt and molybdenum. However, cobalt as an alloying element is costly and not available from a reliable source of supply. In order to avoid the constraints imposed by cobalt, maraging steels without cobalt were developed which contain: EQU Ni 17-26 wt. %, Mo 0.2-4 wt. %, Ti 1-2.5 wt. %, EQU Al&lt;1 wt. %, and optionally some Nb,
with the remainder being Fe and impurities resulting from the processing. Such steels are described, e.g., in Brit. Pat. 1,355,475 and U.S. Pat. No. 4,443,254; both incorporated herein by reference. They enable one to obtain a high tensile strength (on the order of 1800 MPa) and satisfactory ductility, in a metal which is homogenized at elevated temperature followed by cooling and aging.